Injection molding nozzles employing gating designs for hot runner molds typically include hot tip, hot edge gate, thermal shutoff (hot sprue), valve gate, and multiple tip variations of hot tip and hot edge gates. Hot gate designs are generally either externally or internally heated. To reduce molding cycle times and improve gate vestige attributes (i.e., protuberances), the diameter of the gate is usually made relatively small (0.025 inches to 0.060 inches) or (0.064 cm to 0.152 cm). Both externally and internally heated hot tips are presently available that pass the melt over the outside of the heated tip to allow the tip to be positioned closer to the gate and reduce the height of the vestige. Illustrated in FIG. 1, externally heated hot tips 10 normally require a diverted tip member (molten flow being directed from orifices 12, 14 surrounding the tip member) which has a passage 16 for the melt stream to exit around the center 18 of tip member (denoted by arrows) so that the melt can pass over the exterior 20 of the tip member. Moreover, diverted hot tips 10 generally result in a thin annular orifice in the gate portion 21 of the mold cavity 23 with fragile pointed hot tips. The gate portion 21 is generally defined as the connection between the injection nozzle and the molded part. The gate portion must permit enough material to flow into the mold cavity to fill out the part plus such additional material as is required to overcome the normal thermal shrinkage of the part (See generally Joel Frados, Plastics Engineering Handbook of the Society of Plastics Engineering, Chapter 6, "Designing Molds for Injection Molding", page 137, 1976). Thus, diverted hot tips are considered sensitive to contamination, processing temperature, tip wear and damage.
Illustrated in FIG. 2, another conventional hot tip 22 has a passage 24 through to the end 26 of the tip member (referred to generally as a flow through or alternatively straight through hot tip). Nominally this hot tip design, such as the Husky Part No. 532994 made by the Husky Corporation of Bolton, Ontario, Canada, is intended to solve the problems associated with multiple flow fronts affecting part flatness and dimensional control of the molded part, specifically for large thin walled parts molded of crystalline materials like polyethylene or polypropylene. These latter materials are sensitive to flow lines and processing pressures. Flow lines are caused by the union of two or more flow fronts, usually having different temperatures, in the mold cavity. The specific geometry of existing flow through hot tips was not designed to get the end of the tip positioned far enough into the gate portion 28 of the mold cavity 30 for achieving short gate vestige height control. Also, this design was not intended to be used with amorphous materials, such as polystyrene resins. The end of the tip is positioned too far away from the gate portion 28 of the mold cavity 30 and the amorphous resin, during molding process, will tend to solidify before the mold cavity 30 can be filled.
More recently it has become desirable for very high volume molding requiring high cavitation molds with short molding cycle times. High cavitation molds are multiple-cavity molds for use in forming more than one part at a time. These molds primarily utilize hot runner manifolds with hot tip gating for reduced cycle time, increased cavitation and reduced maintenance. One problem associated with these hot tips is gate plugging due to material contamination or particulate matter contained in the melt. Another problem required to be overcome with these devices is gate vestige or protuberances on the finished part due to process condition variations and damage. Moreover, gate string problems, i.e., hair-like plastic material attached to the mold cavity after completion of the mold cycle, are also manifest with such devices due to process temperature and material variations.
Furthermore, the above problems are particularly pronounced with certain molten plastic materials. Particularly troublesome is the use of amorphous resins, like polystyrene, because they do not have a clearly defined glass transition temperature as with crystalline resins. The drive for higher efficiencies, increased process stability and the ability to process post consumer recycled resins motivated the inventors to consider or design alternate hot runner gate designs to minimize or eliminate the above problems.
While there have been numerous improvements in injection molding nozzles, such as described in U.S. Pat. No. 5,046,942 by Gellert, U.S. Pat. No. 4,648,833 by Yamada, and Japanese Patent Application No. 56-139073 by Jiyuuou et al., none have been directed specifically to solving above problems. Jiyuuou et al., Gellert '942 and Yamada '833 each teaches a method of thermally cycling the tip for controlling resin flow. Further, U.S. Pat. No. 2,814,831 to McKee, Jr. describes an injection nozzle with a hemispherical tip especially suited for flat bottom or dish-like articles.